Port plug for a plasma-confining cavity

ABSTRACT

A body defining a cavity confining a plasma in a partial vacuum includes an inwardly tapering conical aperture permitting access to the cavity and a truncated-conical plug sized to engage the aperture in removable-deformable fit engagement a distance along the length of the aperture from the cavity-defining interior walls of the body greater than the resistive skin depth of the body. The plug is further sized to seal the cavity flush with the interior walls of the body and the sides of the plug and the conical aperture are generated at an angle theta relative the vertical such that theta has a value less than tan theta 1/ Mu and greater than tan theta Mu , wherein Mu is the coefficient of static friction between the plug and the body.

United States Patent Lien [ 1 Mar. 21, 1972 PORT PLUG FOR A PLASMA- CONFINING CAVITY [72] Inventor: Neil C. Lien, Evansville, Wis.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission 22 Filed: Dec.2, 1969 21 Appl. No.: 881,356

V [521 U.S. Cl ..l76/l, 220/42, 220/24, 176/3 [51] Int. Cl. ....G2lb 1/00 [58] Field of Search ..220/42 B, 24, 176/1-9 [56] References Cited UNITED STATES PATENTS 3,021,272 2/1962 Baker et a1 ..l76/7 3,281,324 10/1966 Henins et al. ..176/l 3,008,483 11/1961 Beckwith ..220/24 X 3,021,974 2/1962 Watts ..220/42 B X 3,385,466 5/1968 Hook et al. ..220/24 Primary Examiner-Reuben Epstein Attorney-Roland A. Anderson [5 7] ABSTRACT A body defining a cavity confining a plasma in a partial vacuum includes an inwardly tapering conical aperture permitting access to the cavity and a truncated-conical plug sized to engage the aperture in removable-deformable fit engagement at distance along the length of the aperture from the cavity-defining interior walls of the body greater than the resistive skin depth of the body. The plug is further sized to seal the cavity flush with the interior walls of the body and the sides of the plug and the conical aperture are generated at an angle 0 relative the vertical such that 0 has a value less than tan 0 1/;1. and greater than tan 0 ;1., wherein p. is the coefficient of static friction between the plug and the body.

7 Claims, 9 Drawing Figures PMENTEDMARZ'I I972 3,650,893

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PAIENTEUMARZI m2 3 650 893 sum u 0F 4 1/ Z tar/7g PORT PLUG FOR A PLASMA-CONFINING CAVITY CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION Thermonuclear reactions providing thermonuclear power are most likely to occur in high-density plasmas in which the kinetic energy of the particles making up the plasma are at a very high energy level. For such reactions, it is necessary that the plasma be contained at a requisite density and that the particles be excited to a proper temperature with respect to the density with either a continuous or pulsed type of operation.

One of the major problems that has been apparent since the conception of obtaining a thermonuclear reaction is the confinement of a plasma at a sufficiently high density and temperature to enable a thermonuclear reaction to take place. One method of effecting such a confinement is to use magnetic fields in a cusped geometry. The Wisconsin levitated octopole at the University of Wisconsin is a machine which effects such a geometry. It is desirable in this type of machine to provide an access port to the interior of the body of the machine wherein the magnetic fields are generated and the plasma is confined. This port, while providing free access, must not perturb or deform the magnetic fields, since to do so would destroy the plasma-confining characteristics thereof.

Accordingly, it is one object of the present invention to provide an access port to the interior of a body defining a cavity wherein a plasma is confined in a partial vacuum.

It is another object of the present invention to provide an access port in a body defining a cavity wherein a plasma is confined in a partial vacuum by magnetic fields whereby such port does not perturb the confining magnetic field.

It is another object of the present invention to provide an access port in a body defining a cavity wherein a plasma is confined. which access port is characterized by case of closure and opening.

Other objects ofthe present invention will become more apparent as the detailed description proceeds.

SUMMARY OF THE INVENTION In general. the present invention comprises a body defining a cavity confining a plasma including a conical aperture through said body to permit access to said cavity and a truncated-conical plug sized to engage the sides of said aperture in removable deformable contact engagement therewith and seal said cavity flush with the interior walls of said body.

BRIEF DESCRIPTION OF THE DRAWINGS Further understanding of the present invention may best be obtained from consideration of the accompanying drawings wherein:

FIG. 1 is a cross-sectional drawing of an access port according to the present invention in a typical body defining a cavity wherein a plasma is confined by electromagnetic fields.

FIG. 2 is a graphical plot of the insertion and removal characteristics of the port plug illustrated in FIG. 1.

FIG. 3 is a plan view of an aperture for a preferred access port according to the present invention for a body defining a cavity wherein a plasma is confined.

FIG. 4 is a cross-sectional view of FIG. 3 along lines 44.

FIG. 5 is a plan view ofa plug for the aperture of FIG. 3.

FIG. 6 is a sectional view of FIG. 5 along lines 6-6.

FIG. 7 is a sectional view of FIG. 8 taken along lines 7-7 of FIG. 8. illustrating the plug of FIG. 5 inserted in the aperture of FIG. 3.

FIG. 8 is a plan view of FIG. 7.

FIG. 9 is a section of FIG. 8 taken along lines 9-9.

In FIG. I. a cross-sectional view is shown ofa body 10 for a typical machine for effecting plasma confinement using magnetic fields. The current-carrying walls 11 of body 10 define a cavity 12 wherein are disposed current-carrying conductors 14. The currents carried by walls 11 and conductors l4 generate electromagnetic fields 16 within the cavity 12 to form a cusped geometry and provide a containment area 18 in which a plasma may be confined. The flux line 20 is considered a critical flux line, since this line defines the maximum point of the containment area in which the plasma can be stably confined. Beyond this line 20 towards the walls 11 of the body 10 the plasma becomes magnetohydrodynamically unstable.

To provide access to the interior cavity 12 of the body 10 a conically shaped aperture 22 is cut through the top wall 11a of the body 10. Into this conically shaped aperture 22 is inserted a truncated-conical plug 26 sized to engage the sides 28 of aperture 22 in removable-deformable engagement. For the practice of the present invention, the term removable" is defined to provide that the plug 26 may be removed by hand pressure. It is necessary for the successful operation of the confinement of a plasma within the cavity 12 that the plug 26 within the wall 11a provide minimal electrical resistance between the plug 26 and the sides 28 of the aperture 22. This is accomplished using a size relationship to provide deformable insertion of the plug relative the sides 28 of the aperture 22.

Further understanding of the removable structure of the plug 26 relative the aperture 22 may be accomplished by considering two conditions, the insertion of the plug into the aperture 22 and the removal of the plug 26 from the aperture 22. To insert the plug 26 into the aperture 22 a force F is applied normal to the surface of the plug 26. At sliding,

ln =fwhere p. the static coefficient of friction between the plug 26 and the sides 28 of aperture 22,f the force due to friction and F, the normal component of the force. Then at equilibrium sin H f ,(cos H 1 60s (i/(cos H+sin H cos H/ l. sin 6 cos 6+ In order that the plug 26 slide in the aperture 22,

cos 0 sin 6 cos 0/ is greater than I and sin 0 is less than sin 0 cos 6Jpwhereby tan 0 is less than l/u. To provide that the plug 26 removable from the aperture 22 without sticking after insertion,

M F n =ft where j}, the force due to friction, has reversed direction. Then F, sin0=f, c059 and p. sin 6/cos 0 tan 0. Thus, to inhibit sticking of the plug 26 in the aperture 22, it is necessary that the angle 0 be greater than the value tan 0 t. It will be apparent from the foregoing description that for the successful practice and construction of the port plug and aperture according to the present invention it is necessary therefor that the sides 28 of the aperture 22 and the sides of the plug 26 where [L the coefficient of static friction between the plug and the body. Further appreciation of this relationship may be obtained from considering the graphical plots 30 and 32 of tan p. and 1/ 4., respectively. The plots 30 and 32 define four 10 areas 34, 36, 38 and 40, only one of which, area 34, will provide the correct combination, allowing insertion and extraction of the plug within the aperture 22. Where the coefficient of friction and angle 0 are such that their respective values fall within area 36, 38 or 40, insertion of the plug within the aperture 22 will be characterized by either a no-slide or sticking, or both conditions.

As stated previously, in the structure of FIG. 1, the walls 11 and the conductors 14 carry currents which generate the magnetic fields defining the containment area wherein the plasma is confined, since in the plasma device characterized in FIG. 1 operation is effected at low frequencies in the neighborhood of to cycles per second. The currents flowing in the walls 11 of the body 10 penetrate into the body an appreciable distance. If the plug 26 does not provide excellent electrical contact with the sides 28 of aperture 22, then deformation of the currents flowing in the wall lla occurs with resulting deformation of the magnetic fields l6 and possible leakage of the plasma from the containment area 18. With poor electrical contact between the plug 26 and the sides 28 of aperture 22, the current flowing in the wall 11a will deform around the interface of the plug and the cavity and the magnetic fields 16 will follow this same deformation. Thus, the critical flux line 20 will be drawn into the interface between the plug 26 and the sides 28 of aperture 22 to permit the loss of the plasma therebetween. This is avoided in the present invention by making the plug effect an elastic deformable fit engagement with the sides 28 of the aperture 22. With such an engagement and the plug 26 terminating flush with the interior of wall 11a, currents flowing in the wall 11a see only a single conductor and no deformation of the currents and the magnetic fields 16 occurs. To insure that this effect is accomplished, the elastic deformable fit between the plug 26 and the sides 28 of aperture 22 should extend from the interior 29 of the wall 11a outwardly a distance greater than the resistive skin depth of the material forming the body 10.

Turning to FIGS. 3 and 4, a plan and sectional side view is taken for an aperture 42 cut through the wall Ila of the body 10 to form a preferred access port according to the present invention. The aperture 42 comprises an inwardly tapered conical section 44 extending from the interior 45 of the wall lla a distance greater than the resistive skin depth of the material of wall lla, a counterbored section forming a seat 46 and a chamfered portion 48. Into the aperture 42 is inserted the plug 50 illustrated in plan view and sectional view in FIGS. 5 and 6, respectively. The plug comprises a truncated-conical section 52 and shoulder portions 54 and 56, respectively. The shoulder portion 54 is sized to ride without bottoming in the seat 46 of aperture 42 when the plug 50 is inserted to seal the cavity flush with the interior 45 of wall lla. A slot 55 is cut around shoulder portion 54 to permit the insertion of an O- ring 58 to effect a vacuum seal between the walls of the plug 50 and the aperture 42.

The plug 50 and aperture 42 are shown in an engaged position in FIGS. 7 and 8. To effect such an engagement the plug 50 is inserted into the aperture 42 and secured by the securing bolts 60 passing through the apertures 62 in shoulder portion 56 of the plug 50 into the threaded holes 64 in wall lla of body 10. As previously explained. the interior of the body 10 housing the plasma operates in a partial vacuum. To permit the removal of contaminants trapped by the insertion of the plug 50 into the aperture 42, a slot 66 is cut in one side of the plug 50, as shown in FIG. 9. Further, retention screws may be screwed into tapped holes 68 in plug 50 whereby the plug may be held so that a gap is maintained between the conical surfaces 44 and 52 without breaking the vacuum seal of O-ring 58. This allows the conical surfaces 44 and 52 to be vacuum pumped and eliminates high localized bearing stresses between surfaces 44 and 52 as pressure deformations in walls 11 of the body 10 occur from evacuation of cavity 22. After the desired vacuum is reached, the retaining screws in the tapered holes 68 are removed and bolts 60 tightened to the desired torque to effect engagement between the plug 50 and the walls of aperture 42.

Since shoulder 54 does not bottom in seat 46 of aperture 42, an internal restraint is carried between the bolts 60 through the mating conical sides 52 and 44 assuring good electrical sliding pressure through compression and tension hoop strain deformations in plug 50 and wall 11a respectively.

For the above-described embodiment, successful operation has been effected using for the material of plug 50 and the walls 11 aluminum of high electrical conductivity. The following size relationships in inches were used.

Dimension A=6.000 0.000

0.00l B=l.l88:0.00l C=l.230 REF D=0.3l8 0.002

0.000 F=l.230 REF G=l.l88:0.00l

With the aforedescribed dimensions, the insertion of the plug 50 into the aperture 42 is effected with a removable-deformable contact engagement to seal the cavity flush with the interior of wall lla.

Persons skilled in the art will, of course, readily adapt the general teachings of the invention to embodiments far different from the embodiments illustrated. Accordingly, the scope of the protection afforded the invention should not be limited to the particular embodiment illustrated in the drawings and described above but should be determined only in accordance with the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A body of electrically conductive material defining a cavity confining a plasma including a conical aperture through said body to permit access to said cavity, and a truncatedconical solid plug of electrically conductive material sized to engage the sides of said aperture in removable-deformable contact engagement therewith to provide a unitary electrical conductor with said body and seal said cavity flush with the interior walls of said body, the sides of said apertures and plug being generated at an angle 6 relative the vertical such that 0 has a value less than tan 0 equal to 1/,u, and greater than tan 0 equal to p. where u equals the coefficient of static friction between said plug and said body.

2. The apparatus according to claim 1 wherein the exterior of said body is counterbored about said conical aperture, said conical aperture tapers inwardly to said cavity, and said plug includes a portion engageable with said counterbored section of said body.

3. The apparatus according to claim 2 wherein said counterbore extends inwardly of said body to a distance from the interior thereof defining said cavity greater than the resistive skin depth of said body.

4. The apparatus according to claim 3 wherein said cavity is maintained at a partial vacuum further including vacuum sealing means sealably interposed of said counterbored section of said body and said plug portion engageable therewith, and further including means providing evacuative access between said sealing means and said cavity.

5. A body of electrically conductive material defining a cavity confining a plasma in a partial vacuum including an inwardly tapering conical aperture permitting access to said cavity, and a truncatedconical solid plug of electrically conductive material sized to engage said aperture in removabledeformable fit engagement along a length of said aperture from the cavity-defining interior walls of said body greater than the resistive skin depth of said body and provide a unitary electrical conductor with said body, said plug being further sized to seal said cavity flush with the interior walls of said body, the sides of said conical aperture and plug being generated at an angle 9 relative the vertical such that has a value less than tan 6 equal to 1/ and greater than tan 6 equal to ,u, where p. equals the coefficient of static friction between said plug and said body.

6. The apparatus of claim wherein said aperture is enlarged adjacent the exterior surface of said body to provide a seat therein, said plug includes a shoulder portion engageable with said seat, means vacuum sealing said plug shoulder portion to said body at said seat, and evacuative means extending from said sealing means to the cavity defined by said body.

7. The apparatus of claim 6 wherein said shoulder portion is sized to preclude bottoming of said shoulder portion in said seat when said plug seals said cavity flush with the interior walls ofsaid body, and further including a member engageable with said plug and said body to generate an axial force on said plug to engage said plug and the sides of said aperture in said removable-deformable fit engagement and effect compression and tension hoop strain deformations respectively in said plug and said body. 

2. The apparatus according to claim 1 wherein the exterior of said body is counterbored about said conical aperture, said conical aperture tapers inwardly to said cavity, and said plug includeS a portion engageable with said counterbored section of said body.
 3. The apparatus according to claim 2 wherein said counterbore extends inwardly of said body to a distance from the interior thereof defining said cavity greater than the resistive skin depth of said body.
 4. The apparatus according to claim 3 wherein said cavity is maintained at a partial vacuum further including vacuum sealing means sealably interposed of said counterbored section of said body and said plug portion engageable therewith, and further including means providing evacuative access between said sealing means and said cavity.
 5. A body of electrically conductive material defining a cavity confining a plasma in a partial vacuum including an inwardly tapering conical aperture permitting access to said cavity, and a truncated-conical solid plug of electrically conductive material sized to engage said aperture in removable-deformable fit engagement along a length of said aperture from the cavity-defining interior walls of said body greater than the resistive skin depth of said body and provide a unitary electrical conductor with said body, said plug being further sized to seal said cavity flush with the interior walls of said body, the sides of said conical aperture and plug being generated at an angle theta relative the vertical such that theta has a value less than tan theta equal to 1/ Mu and greater than tan theta equal to Mu , where Mu equals the coefficient of static friction between said plug and said body.
 6. The apparatus of claim 5 wherein said aperture is enlarged adjacent the exterior surface of said body to provide a seat therein, said plug includes a shoulder portion engageable with said seat, means vacuum sealing said plug shoulder portion to said body at said seat, and evacuative means extending from said sealing means to the cavity defined by said body.
 7. The apparatus of claim 6 wherein said shoulder portion is sized to preclude bottoming of said shoulder portion in said seat when said plug seals said cavity flush with the interior walls of said body, and further including a member engageable with said plug and said body to generate an axial force on said plug to engage said plug and the sides of said aperture in said removable-deformable fit engagement and effect compression and tension hoop strain deformations respectively in said plug and said body. 